This invention relates to an improvement in a control valve assembly for a hydrostatic fluid power steering system.
Hydrostatic auxiliary power steering systems having at least one rotatable slide valve element operated by a valve shaft extending from a valve body, is already well known. In this type of valve assembly, an outer distributor slide valve is rotatably mounted within a bore of the valve body while an inner reversing slide valve element is rotatably mounted within a bore of the outer distributor valve. The reversing valve is formed in one piece with the valve shaft. The shaft extends externally of the valve body through an exit bore with radial clearance. The annular gap between the exit bore and the valve shaft is sealed at one axial end by a pressure tight, annular sealing gland. The sealing gland includes a resilient O-ring element having a useful life of substantial duration only when there is a relatively small radial clearance between the bore and the shaft. Where the radial clearance is relatively large, the sealing element is destroyed by gap extrusion.
Where one or more slide valve elements are utilized, the valve elements must be mounted within the valve body with a relatively tight sliding fit. However, because of manufacturing imperfections, there is a certain amount of eccentricity between the valve surfaces and shaft and the associated valve bores and shaft exit bore. To avoid jamming between the relatively movable parts, the radial clearance between the valve shaft and exit bore must be made relatively large. However, any enlargement of the radial clearance necessary to avoid jamming often leads to destruction of the sealing element by gap extrusion as aforementioned.
An attempt to solve the foregoing problem is proposed in U.S. Pat. No. 3,785,661 to White. According to the White patent, a relatively large radial clearance is provided between a rotatable shaft and an exit bore in the housing through which it extends forming a gap filled by an annular sealing gasket. The gasket at one axial end is acted upon by pressurized fluid in an operating chamber while its other axial end is supported by an annular disk that extends into the exit bore with little radial clearance. The annular disk is supported by an element seated on the shaft with a tight fit. The support element and the annular disk therefore both undergo eccentric movement during operation and the annular gasket is radially and periodically compressed and elongated. As a result, the annular gasket undergoes a squeezing action at the radially inner edge of the annular disk resulting in damage thereto.
It is therefore an important object of the present invention to provide a steering control valve assembly with a substantial amount of radial clearance between the valve shaft and the exit bore of the valve body without causing damage to the sealing element during operation of the control valve assembly. A further object in accordance with the foregoing object is to provide a control valve arrangement which avoids the seal destruction problem with a relatively simple construction having low production and assembly costs.